本文分析基于Linux Kernel 1.2.13
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作者：闫明
注：标题中的”（上）“，”（下）“表示分析过程基于数据包的传递方向：”（上）“表示分析是从底层向上分析、”（下）“表示分析是从上向下分析。
在博文Linux内核--网络栈实现分析（三）--驱动程序层（链路层）（上）中对网络设备结构，网络设备初始化等函数有了初步认识，并列出了设备的发送和接收函数。
设备接口层会调用函数设备驱动层ei_start_xmit()函数发送数据，这里没有详细分析。
static int ei_start_xmit(struct sk_buff *skb, struct device *dev)
{
int e8390_base = dev->base_addr;
struct ei_device *ei_local = (struct ei_device *) dev->priv;//取出网卡设备的私有数据，和具体的网卡型号有关，在ethdev_init()函数中已经分配空间
int length, send_length;
unsigned long flags;
/*
* We normally shouldn't be called if dev->tbusy is set, but the
* existing code does anyway. If it has been too long since the
* last Tx, we assume the board has died and kick it.
*/
if (dev->tbusy) { /* Do timeouts, just like the 8003 driver. */
........................................
........................................
}
/* Sending a NULL skb means some higher layer thinks we've missed an
tx-done interrupt. Caution: dev_tint() handles the cli()/sti()
itself. */
if (skb == NULL) {//该条件似乎不会发生，这用于处理内核中的BUG
dev_tint(dev);//发送设备中的所有缓存的数据包
return 0;
}
length = skb->len;
if (skb->len <= 0)
return 0;
save_flags(flags);
cli();
/* Block a timer-based transmit from overlapping. */
if ((set_bit(0, (void*)&dev->tbusy) != 0) || ei_local->irqlock) {
printk("%s: Tx access conflict. irq=%d lock=%d tx1=%d tx2=%d last=%d\n",
dev->name, dev->interrupt, ei_local->irqlock, ei_local->tx1,
ei_local->tx2, ei_local->lasttx);
restore_flags(flags);
return 1;
}
/* Mask interrupts from the ethercard. */
outb(0x00, e8390_base + EN0_IMR);
ei_local->irqlock = 1;
restore_flags(flags);
send_length = ETH_ZLEN < length ? length : ETH_ZLEN;
if (ei_local->pingpong) {
int output_page;
if (ei_local->tx1 == 0) {
output_page = ei_local->tx_start_page;
ei_local->tx1 = send_length;
if (ei_debug && ei_local->tx2 > 0)
printk("%s: idle transmitter tx2=%d, lasttx=%d, txing=%d.\n",
dev->name, ei_local->tx2, ei_local->lasttx,
ei_local->txing);
} else if (ei_local->tx2 == 0) {
output_page = ei_local->tx_start_page + 6;
ei_local->tx2 = send_length;
if (ei_debug && ei_local->tx1 > 0)
printk("%s: idle transmitter, tx1=%d, lasttx=%d, txing=%d.\n",
dev->name, ei_local->tx1, ei_local->lasttx,
ei_local->txing);
} else { /* We should never get here. */
if (ei_debug)
printk("%s: No Tx buffers free. irq=%d tx1=%d tx2=%d last=%d\n",
dev->name, dev->interrupt, ei_local->tx1,
ei_local->tx2, ei_local->lasttx);
ei_local->irqlock = 0;
dev->tbusy = 1;
outb_p(ENISR_ALL, e8390_base + EN0_IMR);
return 1;
}
ei_block_output(dev, length, skb->data, output_page);
if (! ei_local->txing) {
ei_local->txing = 1;
NS8390_trigger_send(dev, send_length, output_page);
dev->trans_start = jiffies;
if (output_page == ei_local->tx_start_page)
ei_local->tx1 = -1, ei_local->lasttx = -1;
else
ei_local->tx2 = -1, ei_local->lasttx = -2;
} else
ei_local->txqueue++;
dev->tbusy = (ei_local->tx1 && ei_local->tx2);
} else { /* No pingpong, just a single Tx buffer. */
ei_block_output(dev, length, skb->data, ei_local->tx_start_page);
ei_local->txing = 1;
NS8390_trigger_send(dev, send_length, ei_local->tx_start_page);
dev->trans_start = jiffies;
dev->tbusy = 1;
}
/* Turn 8390 interrupts back on. */
ei_local->irqlock = 0;
outb_p(ENISR_ALL, e8390_base + EN0_IMR);
dev_kfree_skb (skb, FREE_WRITE);
return 0;
其中的dev_tint()函数是将设备的所有缓存队列中的数据全部调用dev_queue_xmit()发送全部数据包。
/*
* This routine is called when an device driver (i.e. an
* interface) is ready to transmit a packet.
*/
//该函数功能:遍历设备的缓冲队列，对所有的数据包调用dev_queue_xmit()函数发送数据
void dev_tint(struct device *dev)
{
int i;
struct sk_buff *skb;
unsigned long flags;
save_flags(flags);
/*
* Work the queues in priority order
*/
for(i = 0;i < DEV_NUMBUFFS; i++)
{
/*
* Pull packets from the queue
*/
cli();
while((skb=skb_dequeue(&dev->buffs[i]))!=NULL)
{
/*
* Stop anyone freeing the buffer while we retransmit it
*/
skb_device_lock(skb);
restore_flags(flags);
/*
* Feed them to the output stage and if it fails
* indicate they re-queue at the front.
*/
dev_queue_xmit(skb,dev,-i - 1);//注意优先级的计算方式，在函数dev_queue_xmit()中优先级若<0则计算pri=-pri-1=-(-i-1)-1=i，
//这样做的目的就是为了得到正确的where值,函数(dev_queue_xmit())中
/*
* If we can take no more then stop here.
*/
if (dev->tbusy)
return;
cli();
}
}
restore_flags(flags);
}
驱动层严格的说不属于内核网络栈的内容，和硬件关系密切，何况这种网卡硬件设备可能已经不用了，这里就没有详细分析，如果对网卡驱动有兴趣可以看一下之前的分析的ARM-Linux下的DM9000网卡驱动的分析，链接如下：